DEPARTMENT OF TERRESTRIAL MAGNETISM. 347 



The axial distance between the centers of the two coils, or the distance be- 

 tween corresponding turns of the spirals, is approximately 15 cm. The insu- 

 lation resistance between adjacent wires is very high. 



The method of measuring the diameters and axial distance of the spiral is 

 described and the results given in tables and curves. The magnetic tests, 

 of three kinds, proving the materials to be satisfactoiy, are also described. 



The theory of the instrument, the method of using it, and the calculation of 

 the error in the constant of the coils due to the construction, as well as of the 

 other errors introduced in the measurement of the horizontal intensity, are 

 given in sufficient detail. 



It is shown that the errors in reading the circle and the telescope scale when 

 sufficiently large angles are used, and the error in the constant of the coil, are 

 quite negligible; and that the only other error necessary to consider, viz, that 

 introduced in the measurement of the current traversing the coils, can also 

 be made entirely negligible. In consequence, the horizontal intensity of the 

 Earth's magnetic field can be determined with an error less than 1 part in 

 10,000, which is all that can be desired. 



The instrumental work done in the shop of the Department, chiefly by 

 Mr. G. H. Jung, instrument-maker, is highly satisfactory. 



The paper gives a brief statement of the results obtained in two series of 

 simultaneous determinations of the horizontal intensity with the sine gal- 

 vanometer and standard C. I. W. magnetometer No. 3, Messrs, Fleming, 

 Fisk, Peters, and the author participating in the first series, and Messrs. Flem- 

 ing, Fisk, and Ives in the second and much more extensive series. The per- 

 formance of the galvanometer and the agreement between the results obtained 

 with the two instruments have been entirely satisfactory. 



On vertical electric currents and the relation between terrestrial magnetism and atmospheric 

 electricity. Louis A. Bauer. Terr. Mag., vol. 25, 145-162 (Dec. 1920). 



To what extent the magnetic forces as observed on the surface of the Earth 

 can be referred to a potential is a subject of paramount interest. The 

 solution of the problem is of great importance, both as regards the constitu- 

 tion of the so-called permanent magnetic field of the Earth and the systems 

 giving rise to the manifold variations to which the terrestrial magnetic field 

 is continually subject. 



Any electric currents circulating above or below the Earth's surface in 

 concentric layers, i. e., parallel to the surface, will give rise to magnetic forces 

 which may be represented by a potential. Electric currents, on the other 

 hand, cutting the Earth's surface, produce, in general, a mixed magnetic 

 system; the horizontal components of such currents give rise to a magnetic 

 potential, whereas the vertical components cause magnetic forces which can 

 not be referred to a potential. As is well known, the test of the existence of a 

 potential is the vanishing of the line-integral of the magnetic force taken 

 around a closed curve, or circuit, on the Earth's surface. If the hne-integral 

 does not vanish and its departure from zero can not be explained by error of 

 observation, or local magnetic disturbances in the region of the circuit, then 

 the existence of a non-potential is revealed; from the magnitude and sign of 

 the integral we may then determine the strength and direction of the electric 

 currents passing perpendicularly through the surface of the region inclosed by 

 the circuit. 



It was deemed desirable to make first a reconnaissance and repeat the 

 author's former computations of line-integrals along latitudinal circuits, 

 using this time magnetic charts, which while not representative of all the 

 available data of the Department of Terrestrial Magnetism are close approxi- 

 mations thereto. Line integrals were also computed for various circuits in 



